Donna M. Baldisseri

Structure of the negative regulatory domain of p53 bound to S100B(betabeta).

A Ca2+ dependent conformational change in dimeric S100B(ββ) is required for it to bind p53 and inhibit phosphorylation of this tumor suppressor in its C-terminal negative regulatory domain. A peptide derived from this region of p53 (residues 367–388) was found to have no regular structure in its native form by NMR spectroscopy, but becomes helical when bound to Ca2+ loaded S100B(ββ). The three-dimensional structure of this complex reveals several favorable hydrophobic and electrostatic interactions between S100B(ββ) and the p53 peptide in the binding pocket, where S100B(ββ) sterically blocks sites of phosphorylation and acetylation on p53 that are important for transcription activation.

Solution NMR structure of S100B bound to the high-affinity target peptide TRTK-12.

The solution NMR structure is reported for Ca(2+)-loaded S100B bound to a 12-residue peptide, TRTK-12, from the actin capping protein CapZ (alpha1 or alpha2 subunit, residues 265-276: TRTKIDWNKILS). This peptide was discovered by Dimlich and co-workers by screening a bacteriophage random peptide display library, and it matches exactly the consensus S100B binding sequence ((K/R)(L/I)XWXXIL). As with other S100B target proteins, a calcium-dependent conformational change in S100B is required for TRTK-12 binding. The TRTK-12 peptide is an amphipathic helix (residues W7 to S12) in the S100B-TRTK complex, and helix 4 of S100B is extended by three or four residues upon peptide binding. However, helical TRTK-12 in the S100B-peptide complex is uniquely oriented when compared to the three-dimensional structures of other S100-peptide complexes. The three-dimensional structure of the S100B-TRTK peptide complex illustrates that residues in the S100B binding consensus sequence (K4, I5, W7, I10, L11) are all involved in the S100B-peptide interface, which can explain its orientation in the S100B binding pocket and its relatively high binding affinity. A comparison of the S100B-TRTK peptide structure to the structures of apo- and Ca(2+)-bound S100B illustrates that the binding site of TRTK-12 is buried in apo-S100B, but is exposed in Ca(2+)-bound S100B as necessary to bind the TRTK-12 peptide.